Load-regulating means for grinding machines



Sept. 30, 1930. F, A. PESCHL 1,777,220'

LOAD REGULATING MEANS FOR GRINDING MACHINES Filed March 7, 1928 3 Sheets-Sheet l I N VEN TOR.

ATTORNEY}:

Sept. 30, 1930. I

LOAD REGULATING MEANS FOR GRINDING MACHINES F. A. PESCHL 1,777,220

Filed March 7, 1928 3 Sheets-Sheet 2 INVENTOR.

ATTORNEYS,

Sept. 30, 1930. F. A. PESCHL ,2

LOAD REGULATING MEANS FOR GRINDING MACHLNua Filed March 7, 1928 3 Sheets-Sheet 3 INIILVTOR.

' A771 wiys.

Patented Sept. 30, J

T UNITED STATES PATENT oar-Ice FBA NK A. PESC'HI, OI JACKSON HEIGHTS, NEW YORK LOAD-BEGULATING'MEANS FOB GRINDING MACHINES Application filed March 7, 1928. Serial No. 259,878.

- stone carried by a rotary driving shaft, to

which stone the wood in the form of short logs or billets is fed through a pocket or magazine, the wood feeding means being in some cases in the form of hydraulically operated pressure shoes or plungers, and in other cases endless traveling chains equipped with spurs or lugs to engage the wood columns are employed which chains may be driven in various ways, as for instance, by an electric motor, or by hydraulically driven means, or by other forms of power.

Due to the changing conditions incident to the operation of the machines and the grinding action, the character of the stone structure and. its grinding surface, the nature of the wood being ground and the changes in the area of the sheared surfaces as the grinding operation progresses, and other conditions, the resistance offered by the pressure of the wood column on the stone will vary and will 7 result in a non-uniform grade of pulp, overload of the stone tending to the creation of dangerous stresses, underload of the stone resulting in reduced capacity, and other ob- Jections unnecessary to here enumerate.

It is desirable that in such variations in the resistance offered by the wood to the rota- 'tion of the stone, means be provided for controlling the driving load applied to the stone,

' so that the driving load will be consistent with the conditions of grinding and will not in the one case be greater than is necessary and thereby create stresses on the stone, or in another case be less than is necessary and thereby reduce the capacity of the stone. For instance, the grinding surface of the stone may become relatively smooth by wear, and un er a given pressure of wood against the stone less resistance to the rotation of the stone will be offered than in the case of a Y to wit in safe limits, has been to adjust the load V by the drivin rough or sharp stone, and manifestly an unchanging driving load. on the stone under such chan 'ng conditions would result in the one case 0 a load too high for the stone ma: terial, and in the other case in a load so small 50 that the full ca acity of the stone would not be secured, It ollows from this that the only safe method in operating the grinder in order rovide for maximum load fluctuations so that the stone will be subj ected to stresses considerably below that which it might be well adapted to safely carry, but under such method of operation loss in capacity results.

It is the aim of the present invention to Q0 overcome these objections and to re ate the driving load on the stone to meet t e changing conditions which arise in practice, so that the driving load applied to the stone will be that best suited for its eflicient operas ation under conditions of maximum capacity and safety, and in carrying out this object the invention in one of its aspects consists of so forming the connection between the motor and stone, that a relative motion of the two will be er'mitted in accordance with variations in t e resistance ofiered by the wood to the rotation of the stone, and combining therewith means controlled by said relative motion b on the stone may regulated in accordance with said variations in resistance.

the accompanyin drawings I have shown the invention applie in connection with a plurality of stones both driven by the one motor, and in such an arrangementandinaccordance with the present invention from another aspect of the same, means are provided whereloads'on the stones will be regulated in 'vidually according to the changes in resistance which the pressure of the wood columns offer to the rotation of the .0

which the driving load 15- respective stones, without in any way affecting the driving load on any other stone or stones.

Means and mechanisms of difierent forms and operating in different ways may be provided in bringing about this result, and in the accompanying drawings showing one form of the mechanism by way of example, the impulse for controlling one of the stones, that farthest away from the stone driving motor, is obtained by interposing in the shaft and at a point between the stones, a yielding.

structure by which the two stones are driven crease or decrease the wood pressure on the stone and consequently the resistance ofltered to the rotation of the same ;.but it will be obvious that such controlling impulse may be employed to operate valves controlling the supply of pressure fluid to hydraulic cylinders in cases where the wood feeding means is operated by a hydraulic motor or-mechanism instead'of by an electric .motor.

- The controlling impulse for the other stone.

is, in the form of the invention illustrate-d,

obtained from a power line supplying current to an electric motor driving the shaft structure of the two stones, variations of said current acting thru the medium of electrically operated mechanismto reduce the pressure of wood on the stone in the event-of an increase of driving load thereon, and acting to increase the pressure of the wood on the stone in the event of a decrease of the driving load, such variations in the pressure of the wood against the stone being'eifected by increasing or decreasing the torque of the electric motor driving the wood feeding means when such form'o'fj driving power. is

employed, or by operating valves which will control the supply of pressure fluid to a hydraulic power cylinder when such .form of power is employed to feed the wood.

' This impulse obtainedfrom the power line of the grinder driving'motor, is a total impulse, i. e., the impulse from both stones, but the" electrically operated controlling mechanism acts on the feedmechanism of that stone only which is nearest the stone driving, mo-i tor, since means are provided which in the,

controlling action will deduct from the total which action impulse, the impulse Efromthe remote stone,

will be dwcribed inv detail in,

the specification to follow.

' As far as Ia n aware,'there been no means in cases where a plurality of grindstones are driven from a common source of,

power, as where both stones have been mounted on a unitary shaft structure, to regulate the driving load on the stones individually, the stones in such prior constructions being subjected to the same comdriving-loads on a plurality of stones individually and independently of each other according to the conditions incident to the operation of the individual stones where both stones receive their driving power from a common source, and I believe myself to be the first under such conditions to regulate the driving loads on the stones individually and independently of each other to meet the different conditions which may obtain with reference tothe variations in the resistance offered by the wood columns to the rotation of the respective stones; and in the description to follow the invention will be described in detail, and the novel features thereof will be pointed out in the appended claims.

In the accompanying drawings:

Fig. 1 is a perspective view of two wood grinding machines, showing the common driving shaft structure equipped with a yielding elgnent to produce the load controlling impulse, in accordance with the invention. A

Fig. 2 is a diagrammatic view of one embodiment of the invention showing the electrically operated mechanism and circuits, the stone driving motor, and the motors for operating the wood feeding means controlled by said electrically operated mechanism.

Fig. 3 is a cross section throughthe flexible coupling of the driving shaft structure on the line 3-3 of Fig. 2.

Fig. 4 is-a view similar to Fig. 2, of a modification of the invention. 1

Referring to Figs l, 2 and 3: The wood grinders 1 and 2 to which he nvention is shown by way of example as applied, comprise a plurality of grindstones ".3 and' l, over each of which is a. pocket or magazine 5 to hold the wood to be ground,

"through which pockets extend" the inner stretches of endless traveling feeding chains 6 furnishedlw'ith suitable lugs or spurs to engagethe columns of wood in the maga-' zines. These chains travel at their upper ends around sprocketwheels 7 mounted on themagazme frames, and at their lower ends around sprocket wheels 8 mounted on the 'magazine frames adjacent the grindstones, and the chains-are driven, in the present in-- stance, by ele'ctric motors, a motor 9 for the grinder ane a motor 10 forthe grinder'2,

loo

which motors are geared to the lower sprocket wheels of the feeding chains of the respective motors by suitable reduction gearing 11, 12 and 13.

The grindstones are mounted on a driving shaft structure consisting of a shaft section 14 to which the stone 3 is fixed, and an alined shaft section 15 to which the stone 4 is fixed, which shaft sect-ions are, in accordance with the invention, yieldingly coupled together by a coupling 16, and is driven by a three p ase synchronous electric motor 17 supplied with current from a power line 17 connected to the shaft section 14 as The coupling 16 comprises a head or disc 18 fixed to the end of the shaft section 14 and a similar head or disc 19 fixed to the adjacent end of the shaft section 15 which heads are provided at diametrically opposite points respectively with axially extending lugs 20 and 20 facing each other in a circumferential' direction, compression springs 21 being arranged betweenv adjacent lugs. The springs are of a strength which will permit the driving, force applied to the shaft section 14 to be transmitted to the shaft section 15, the springs in such transmitting action being compressed more or less according to the resistance offered by the shaft section 15 to the rotation of the shaft section 14.

Such relative displacement of the heads of the two shaft sections is utilized in ac- 1 cordance with the present invention, to furmsh an impulse by means of which, controlling conditions will be set up which will regulate the pressure of the wood on stone 4 according to the conditions incident to the operation of the stone, to the end that this stone will receive a driv'ng load best suited for its operating condi ions.

The impulse furnished by the relative movements of the two'heads of the coupling in compressing the springs, acts to vary the strength of an electric current-in a circuit including an induction element carried by the coupling head 18 and consisting of a coil 22 which in the movements of the two heads relatively to each other will move towards or away from an iron core 23 carried by the head 19, the movement of the coil away from core inducing an increased ampere flow in the circuit, and the movement of the coil towards the core causing a decreased ampere flow of current.

The current is supplied to the induction' element from an A. C. supply line 24 through conductors which and the variations rent are employed erated mechanism in' the strength of the ourto control electrically opwhich on an increase of the current strength, as-when the resistance to. the rotation of the stone increases, will weaken the torque of the feed motor thereby decreasingthe pressure of the wood on vthe arrangement of the shown in Fig. 2.,

will be described later on,

stone and consequently decreasing the resistance of the stone to rotation, and a decrease of the current strength will bring about a' reversal of these controlling actions.

These operations are brought about by the parts shown diagrammatically in Fig. 2 Where it will be seen that the feed motors 9 and 10 are supplied with current respectively from generators 25 and 26 coupled together and driven by an electric motor 27 supplied with current from a suitable source of power as by the three phase power line 28. The armature of the generator 25 is connected in series with the armature 29 and field 30 of the feed motor 9 by conductors 31 and 32; and the armature of the generator 26 is connected in series with.

ductor 43 to the negative conductor 44 of the supply line. Likewise the field 45 of the generator 26. is connected to said supply line, the current flowing from the positive conductor 39 through conductor 46, to the generator field 45 and leaving said field, flowing through conductor 47, thence to adjustable resistance 49, and from the resistance by conil uctor to the negative conductor of the me. I

A circuit constituted by conductors 51 and 52 is connected around the resistance 42 to shunt'the same, which circuit is controlled by a relay 53 supplied with current from the supply line 38, the said current flowing through conductor 54 leading from the positive conductor of the line, thence to the coil of the relay, thence by conductor 55 to contact point 56 then to contact point 57 carried by a pivoted lever 58, and thence by conductor 59 to the negative conductor of the supply line. The control of the shunt circuit by A circuit constituted by conductors 60 and 61 i3, connected around the resistance 49 to shunt the same, and this shunt'circuit is controlled by a relay 62 supplied with current deenergized the shunt.

from'the supply line 38 by the following conductors: Starting with the supply line conductors 39, the circuit flows by conductor 63, to contact point 64 on a pivoted lever 65, thence to contact point 66, and by conductor ductor 39 of the line, adjustable rheostat 105,

'two coils 73' and 74 67 to the coil of the relay, and leaving the coil, the current flows to the negative conductorof the supply line by conductor 68.

The relay 62 acts similarly to the relay 53, in controlling its associated shunt circuit, in that the shunt circuit will be closed when the relay coil is energized, and will be opened when the relay coil is deenergized.

The two pivoted levers 58 and 65 are pivoted between their ends respectively on pivots 69 and 70, and the said levers have connected respectively to their inner ends, cores 71 and 72 acted on by solenoid coils,'a coil 73 for the core 71 and a coil 74 for the core 72, and these coils are connected in series with the circuit including the coil 22 of the induction element before referred to, which circuit may be traced as follows beginning with the conductor 80 of the A. C. supply line 24 before referred to, fiConductor 81 leading from the conductor rheostat ,82, conductor 83, trip coil 84, conductor 85, contact point 85*, contact point 85*", conductor 86, slip ring 87 on the shaft section 1,4.conductor 87 coil 22 of the induction element, conductor 88, slip ring 89 on the shaft section 14. conductor 90, coil 91 of ammeter 92 for indicating the load on the stone 3. conductor 93, a'mmeter 94 for indicating the load on stone 4, conductor 95, conductors'96 and 97 leading respectively to the two coils 73 and 74. conductors 98 and 99 leading respectively from said coils, and conductor 100 connected to conductor 101 of said supply line 24.

According to the strength ofthe current flowing through the circuit just traced, the

cores 71 and 72 to subject the inner ends of the two levers 58 and 65 to more or less downward pull. the pull of lever 58 tending to close Th the circuit of nelay 53 through contact points 564 and 57, and the pull on lever 65 tending to open the circuit of relay 62 at contact points 64 and 66.

The pull of the coil 74 on the inner end of lever 65 is resisted by a coil 102 acting on a core 103 on the opposite end of the lever, which coil is included in a circuit fed from the D. C. supply line 38 and constituted by conductor 104 leading from the positive conconductor 106, coil 102, and conductor 107 connected with the negative conductor 44 of the supply line.

The pull of the coil 73 on the inner end of the lever 58 is resisted by a coil 108 acting on a core 109 connected to the opposite end of v the lever,'whi'ch coil 109 is connected in acir-- cuit including a transformer coil 110 energized from one of the leads of the three phase power line 17 driving the motor 17.1 This circuit, starting at one end of the coil 110 is constituted by a conductor- 111 leading to coil 108, conductor 114, coil 115 opposing coil 80, adj ustable:

will act through their b 91 of the ammeter 92, and conductor 116 connecting the ammeter coil with the other end of coil 110. A core 71" is connected in tandem with the core 71 and is acted on by a coil 73*, which is fed from the D. C. suppl line 38 by a circuit including an adjustable rheostat 112. This coil 73 introduces a load setting on lever 58, and consequently on stone 3. v

In the. actions of the hereinbefore described system in controlling the individual driving loads on the two stones the operation will be as follows: 1

If the load on stone 4 increases, due to increased resistance to the rotation of the stone, the springs in the shaft coupling will be compressed, and. the coil 22 of the induction element will move away from the core 23, thereby opening the air gap between them to a definite position. This will cause an increase of the current flow in coil 74, which will through the medium of core 72, pull down on the inner end of lever 65." Such pull on the lever will overcome the pull of coil 102 on the opposite end of the lever, the force of the pull of this latter coil having been previously set by the adjustable rheostat 105 in accordance with the load which it is intended that the stone 4 should carry. The downward pull of the inner end of lever 65 will open the relay circuit at the contact points 64 and 66, and the coil of relay 62 being deenergized, the shunt circuit around resistance 49 will be opened thereby inserting said resistance in the circuit including the field45 of generator 26. This will reduce the generator current flowing to the feed motor 10 of stone 4, therey decreasing the force urging the Wood column against thestone, ing'load on the stone to fall oif or decrease.

e springs of the coupling will now expand, and the coil 22 of'the induction element will approach the core 23, thereby lessening the air gap between the two and and causing the drivbe closed at contact points 64' generator current flowing to the feed motor 10,; and will. increase the force urging the Wood against the'stone, and accordinglyincrease the driving load on the stone accordmg to the requirements ogt'its proper grindin action. r Y r the regulating action of the system 'on the 'ving load of stone 3, the operation 'is as follows: In the'event for instance,"-of an increase of driving load, due to .an increase of resistance to the rotation of the stone. Under such conditions, the current through coil 110 will increase in strength and coil 108 will pull down on lever 58 with a force which will dominate the pull of coils 73 and 73 thereby opening the circuit through the coil of relay 53, which will open the shunt circuit around the resistance 42, thereby inserting said resistance in the circuit including the field 37 of enerator 25. This will reduce the strength 0% the current fed by the generator to the feed motor 9, and will reduce the pressure of the wood column on the stone, and this will cause the driving load on the stone to fall off. Due to such reduction of the driving load, the pull of coil 108 on lever 58 a crease the strength of current flowing to the feed motor 9, and will increase the pressure of the wood against the stone, thereby again increasing the driving load'applied to the stone.

In this action it will be understood that, due to the fact that the coil 73 acting on one end of lever 58 receives the same amount of current as coil 74 acting on lever 65, the pull of coil 108 which is in accordance with the driving load applied to bOthStOIlGS, is counteracted by the pull of coil 73 on the other end of lever 58, and also the pull of the. load setting coil 7 3" which coil 73 pulls in accordance with the driving load on stone 4. It follows from this that lever 58 will act on the difference between the total driving load on both stones and the driving load on stone 4 plus pull of load setting coil 73*, which difference represents the driving load on 4 stone 3.

A very important function is performed by the coil 73 in addition to the action described, for if the stone 4 should fail to take its load fully or not at all, due to a bound chain, or jam of the wood in the pocket, or due to a break in the feeding mechanism, or other causes, then'the pull of coil 73 becoming weaker, will automatically affect the action of lever 58 by resetting its load adjustment to a corresponding lower resistance, so that stone 3 will never receive a greater driving load than that .for which it is set.

It might happen that one or both of the springs in the spring coupling will break, which would cause the air gap between the coil 22 and core 23 to open wide and induce a high current in the induction circuit. While such action would reduce the driving load on stone 4-in the manner hereinbefore described, the induction element would not become reset, and the flow of the current therethrough would remain high and coil 71 would under these conditions move lever 58 high load. In order to prevent this, the trip coil 84 in the induction circuit as before reterred to, is provided, which is so constructed that it will auomatically open said circuit if the flow of the current therethrough reaches a predetermined maximum value, such as would'happen in the event of the breakage of one or both of the springs of the coupling.

In Fig.4 the invention is shown applied to grinders in which the wood is fed to the stones by hydraulic means, a hydraulic cylinder 120 for stone 3, and a similar cylinder 121 for stone 4, which cylinders are equipped with pistons and plungers (not shown) adapted to act on the wood in the magazines" or pockets and force the same against the stones.

Fluid pressure is supplied to the cylinders from a high pressure main 122 through. branch pipes 123 leading tothe respective cylinders, in which branch pipes controlling valves 125 are applied. The valves are operated by electric motors 126 through the medium of beams 127 pivoted between their ends on nuts 128 on vertical screws 128 geared to the respective motors. The motors are supplied with current from a power line 129 through circuits connected to said line, in. which circuits switches 1,30 and 131 are connected. The switches are operated by coils 132 and 133, and the said switches are adapted when closed to pass the current from the power line in opposite directions respectively through the motors so as to cause the motors to open or close the controlling valves and thereby admit or cut oil pressure to the power cylinders. The coils of the switches are included incircuits connected with a supply line 134, and are controlled by contact levers 58 and 65 which levers are controlled in turn by the induction element incorporated in the yielding coupling 16 on the shaft structure similar to the arrangement in the first form of the invention described.

This mechanism will operate on an increase "or decrease of resistance offered to the stones individually considered, to automatically control the fluid pressure to the cylinders so as to reduce the pressure of the wood on an increase of reslstance, and increase-the pressure of the wood on a decrease of resistance, thereby regulating the loads to the stones in accordance with the variations of resistance.

It is obvious that instead of hydraulic cylinders and pistons for forcing the wood against the stone, the wood may be fed-by a hydraulic motor, the operation of which would be controlled by the same mechanisminto a position setting the stone 3 for a very here shown for controlling valves in the fluid pressure supply pipes.

It will be understood that while the invention in so far as its applicability to a plurality of stones is concerned, has been illustrated in connection with two stones, it is obvious that a greater number of stones may be driven from the single driving motor, and the driving loads thereon regulated individually, without departing tom the spirit of the invention. r

In the foregoin description and accompanying drawings have set forth my invention in the particular detailed form and arrangement of parts which are suitable for reahzing the advantages of the invention, but it will be understood that these details may be variously changed and modified without departing from the limits of the invention; and further it will be understood that the invention is not limited to any particular form or construction of the parts except in so far as such limitations are specified in the claims. I

Having thus described my invention what I claim is:

1. In combination with a grindstone, a

driving motor therefor, means for feedingwood to be ground to the stone, a driving connection between the 'motor and stone formed to admit "of a relative motion of the two in accordance with. variations in the re-' sistance ofiered by the wood to the rotation of the stone, and means controlled by, said relative motion for regulating the driving load on the stone.

2. In combination with a grindstone, a

driving shaft structure on which said stone 1s mounted, a motor for rotating the shaft structure to drive the stone, means for feedng wood to be'ground to said stone, a yielding coupling element in said shaft structure permitting of a circumferential movement of the stone relative tothe motor in accord,-

ance with changes in resistance oflered to the rotation of the stone by the wood, and means controlled by said. relative movements for regulating the driving load on the stone in accordance with said changes of resistance.

3. In combination with a grindstone, a driving shaft structure on which said stone ,is mounted, a motor for rotatin'g'said shaft structure to drive the stone, means for feed- ,;ing wood to be ground to said stone, a yielding coupling element in-the shaft structure consistingflof two members movable relatively circumferentially in accordance with changes in resistance offered to the rotation of the stone by the wood, an induction element comprising a coil carried. by one of said members and a core carried by the other member, a circuit including said coil and supplied with electric current from a supply element for re line, and means actuated by said induction wood againstt e stone in accordance with lating the pressure of the mechanism, a generator for supplying current to the motor, means for driving the generator, a circuit supplied with current from a supply line and connected in series with the field of the generator, a resistance included in said circuit, a circuit connected around said resistance and adapted when closed to shunt the resistance, a yielding coupling element in the shaft structure con-- sisting of two members movable circumferentially in accordance with changes in resistance olfered by the wood to the rotation of the stone, an induction element comprising a coil of one of said members and a core on the other, a circuit including said coil and supplied with current from a supply line, and means actuated by said induction element for controlling said shunt circuit.

5. In combination with a grindstone, a driving shaft structure on which saidstone is mounted, a motor for rotating said shaft structure, to drive the same, mechanism for feeding wood to be ground tothe stone, an electric motor for operating the feeding mechanism, a generator for supplying current to the motor, means for driving the generator, a circuit supplied with current from a supply line and connected in series with the field of the generator, a resistance included in said circuit, a circuit connected around said resistance, and adapted when closed to shunt the resistance, a relay for controllingv said shunt circuit andsupplied with current from said supplyline, a yieldingcoupling element in the shaftstructure consisting of two members movable circumferentially relatively in accordance with changes in resistance offered by the'wood in the rotation of the stone, an induction ele- -ment comprising a coil on one of said members and a core on the other member, a circuit including said coil and supplied with current from said supply line, and means actuated by said induction element for controlling the relay circuit.

6. In combination with a grindstone,

means for feeding wood to be ground to the stone, a motor for driving theistone, a connection between the motor and the stone formed to permit of arelative motion of the two inaccordance with changes'in resistance oifered by the wood to the rotation of the stone, an induction element excited from a supply line and subject to changes in current flow by said relative movements of the grindstone, and means controlled by the changes in {connection between the connection between current flow in the induction element for regulating the pressure of the wood against the stone and thereby regulating the driving load on the stone.

7 In combination with a grindstone, mechanism for feeding wood to the stone to be ground, a motor for driving the stone, a motor and the stone formed to permit of a relative motion of the two in accordance offered by the wood to the rotation of the stone, an electric motor for drivingthe feeding mechanism, a generator for supplying current to the motor, means for driving t enerator, a circuit connected in series with the field of the generator and supplied with current froni a supply line, said'circuit including a resistance, a circuit connected around said resistance and adapted when closedto shunt the resistance, an induction element excited from a second supply line and subject to changes in current flow by said relative movements of the grindstone, means actuated by said induction element for controlling said shunt circuit, and means acting in opposition to the induction element in its control of the shunt circuit.

8. In combination with a grindstone, mechanism for feeding wood to the stone to be ground, a motor for driving the stone, a the motor and stone formed to permit of a relative motion of the two in accordance with changes in resistance offered by the wood to the rotation of the stone, an electric motor for driving the feeding mechanism, a generator for supplying current to the motor, means for driving the enerator, a circuit connected in series with the field of the generator and supplied with current from a supply line, said circuit including a resistance, a circuit connected around said resistance and adapted when closed to shunt the resistance, an induction element excited from a second and subject to changes in current flow by said relative movements of the grindstone, means actuated by said induction element for controlling said shunt circuit, and electrical means operated from said first mentioned supply line and acting in opposition to the induction element in its control of said shunt circuit.

, 9. In combination with agrindstone,mechanism -for-feeding wood to be ground to the stone;

' nection between the motor and stone formed to permit of a'relative motionpfthe two in accordance with changes-in resistance ofiered by the wood to the rotation of the stone, an electric motor for driving the feediiiimechanism, a generator for supplying current to the motor, means for driving the generator, a

circuit connected in series with the field of the generator and supplied with current "from a supply line, said circuit including a resistwith changes in resistance closed to shunt the resistance,

supply line a motor for driving the stone, a conresistance offered by the W00 ance, a cireuitconnected around said resistance and adapted when resistance, an induction element excited from a second supply line and subject to changes in current flow by said relative movements of the. grindstone, a contact lever controlling said'shun't circuit, a coil included in the induction circuit and acting on said lever to open the shunt circuit, and a second coil supplied with current from said first mentioned supply line and acting on said contact lever in opposition to the-first coil, to close the said shunt circuit.

a 10. In combination with a grindstone, mechanism for feeding wood to the stone to be ground, a motor'for driving the stone, a connection between the motor and stone formed to permit of a relative motion of the two in accordance with changes in resistance offered bythe wood to the rotation of the stone, an electric motor for driving the feeding mechanism, a generator for supplying current to the motor, means for driving the generator, a circuit connected in series with the field of the generator and supplied with current from a supply line, said circuitincluding a resistance, a circuit connected around said resistance and adapted when an induction element excited from a second supply line said relative movements of the grindstone, a movable member for controlling said shunt circuit, a coil included in the induction circuit and acting on said movable member to open the shunt circuit, a second coil acting on the movable member to close the shunt closed to shunt the and subject to changes in current flow by Too circuit, a circuit supplied with current from the first mentioned supply line and includ 12. In combination with a plurality of grindstones, means for driving said stones rom a common source of power, means for feeding wood to be ground to the respective stones, and means controlled by variations in resistance offered by the wood to the rotation of the respective stones, for automatically regulating the driving loads on the stones individually and independently of each other.

13. In combination with a plurality of grindstones, means for driving said stones from a common source of power, means for feeding wood to be ground to the respective stones, and means actuated b an increase ofto the rotation of either stone each in accordance with and causing an increase of the driving load on either stone, for automatically reducing the resistance so increased independently of that of the other stone and restoring the driving load of the involved stone to its former value.

' 14. In combination with a plurality of grindstones,

rom a Common source of power, means for feeding wood to be ground to the respective stones, and means actuated by a decrease of resistance oifered by the wood to the rotation of either stone and causing a decrease of driving load on the stone, creasing the resistance so reduced independently of that of the other stone and restoring the driving load on the involved stone to its former value.

15. In combination with a plurality of grindstones, means for driving said stones from a common source of power, means for feedingwood to be ground to the respective stones, and means controlled by changes in the resistance offered by the wood to the rotation of the respective stones and causing corresponding changes of the driving loads on the stones, for automatically varying said resistances individually and independently of other in accordance with the changes thereof, and thereby restoring. the driving loads on the i dividual stones to their former values. v

16. In, combination with a plurality of grindstones and means feeding wood to be ground to the respective stones, automatically operated self-regulating means controlled by variations of the resistance offered by the wood to the rotation of the respective stones, for regulating the driving loads on the individual stones independently of each other in accordance with said changes of resistance.

17. In combination with a plurality of grindstones, a driving shaft structure on which said stones are mounted, means for rotating said shaft structure to drive the stones, means for feeding wood to be ground to the respective stones, a yielding coupling element in said shaft structure between the stones permitting of a relative circumferential movement of the stones in accordance with changes of resistance offered to the rotation of one of the stones by the wood, and means controlled by said r'elativemovement for regulating the driving load on said stone independently of the load on the other stone, said changes of resistance.

18. In combination with a grindstones, a driving shaft structure on which said stonesare mounted, means for rotating said shaft structure to drive the stones, means for feeding wood to be gro nd to the. respective stones, and a yielding -oupling element in the shaft structure between the stones permitting of a relative circumferential movement of said stones in accordmeans for driving said stone s for automatically in- 'tive generators,

plurality of of the stones, said ance with changes of resistance offered to the rotation of one of the stones by the wood, and means controlled by said relative move ments for regulating said resistance in accordance with the changes thereof, so as thereby to maintain the driving load on said stone at its proper value independent of the driving load on the other stone. 19. In combination with a plurality of grindstones, a driving shaft structure on which said stones are mounted, a motor connected with said shaft structure and supplied with current from a power line, mechanisms for feeding wood to be ground to the respective stones, electric motors for driving the said feed mechanisms, generators for supply ing current to the feed motors, means for driving said generators, circuits supplied with currentfrom a supply line and connected in series with the fields of the respeceach of the circuits including a resistance, shunt circuits connected around the respective resistances and adapted when closed to shunt said resistances, movable contact members for controlling said shunt circuits, coils acting on the respective members and operating when energized to close one of *1ng on one of the contact members to close the shunt circuit ,controlled by it, said coil being supplied with current from said first mentioned supply line, a coil acting on the other contact member to open the circuit controlled by it and supplied with current from the power line, and means acting on said last mentioned contact member to supplement the. action of its closing coil in accordance with the pull of the closing coil on the other contact member. i

20. In combination with a plurality of grindstones, a driving shaft structure on which said stones are mounted, a motor connected with said shaft structure and supplied with current from a power line, mechanism forfeeding wood to be ground to the respective stones, power means for driving the respective feed mechanisms, circuits connected with a supply line for controlling said power means, movable contact members for controlling said circuits, coils acting on said contact members, an induction element supplied with current from a circuit connected to a' supplyline and subject to changes in current flow in accordance with changes in resistance offered by the wood to the rotation of one induction circuit including said coils, a coil included in a circuit connected witha supply line and acting on one of said contact members and tending to move and tendin 

